1. Field of the Invention
The present invention relates generally to methods and apparatus for sorting particles using fields of force and more specifically relates to methods and apparatus for sorting small particles on the basis of a sensed parameter using a positive electric field.
2. Background of the Prior Art
It has long been known that the trajectory of a charged body moving in a uniform field will be deflected if a second field is generated orthogonal to the first field.
U.S. Pat. No. 2,646,880, for example, shows an apparatus that accelerates small particles uniformly under the influence of gravity and sorts them with an electric field. In this apparatus the small particles have a mass that responds to a gravity field. An electrode places an electric charge on the mass in response to a signal from a photocell and the charged particle is then acted on by the electric field emanating from a plurality of charged plates whose field intensity is constant. The charged particles are deflected into a collecting dish while the uncharged particles fall undeflected into another collection area.
Functionally, the prior art teaches the charging of small particles in response to at least one observed parameter of the particle. The particle is then allowed to fall under the influence of gravity between a plurality of charged plates which deflect its path.
Alternatively, the amount of charge placed on the particle can be varied as an analog of one or more observed parameters (see U.S. Pat. No. 3,380,584, issued Apr. 30, 1968 to Fulwyler); the charge can be self-induced by the particle's movement (see U.S. Pat. No. 3,656,171, issued Apr. 11, 1972 to Robertson); a plurality of particle parameters may be sensed (see U.S. Pat. No. 3,710,933, issued Jan. 16, 1973 to Fulwyler, et al.); or the polarity of the charge placed on the particles may be made a function of the sensed particle parameter (see U.S. Pat. No. 3,826,364, issued July 30, 1974 to Bonner, et al.).
The rapid development of these small particle sorters has accompanied the rise of a need to sort biological objects such as cells and chromosomes. The usefulness of such sorters and a good description of the parameters used by them to discriminate between particles may be appreciated by reading "A New Multiparameter Separator for Microscopic Particles and Biological Cells" in Volume 44 of the Review of Scientific Instruments (September 1973 at page 1301). This reference also discusses the electrical control technology that is well-known by those skilled in the prior art for controlling the prior art's charging electrodes.
An excellent overview of the prior art is found in "Fluorescense-Activated Cell Sorting" in Volume 234, number 3 of Scientific American (March 1976 at page 108).
All of the methods and apparatus taught by the prior art contain fundamental limitation that greatly reduce their usefulness in sorting biological materials. This is a limit on sorting rate that is caused by the use of at least one electrode to charge the particle followed by at least two other electrodes which develop electric fields to deflect the particle.
A particle, according to the prior art, must move through a charging field, which takes time, be transported to a displacing field, which takes more time, and then pass through a displacing field, which takes still more time. If more than one particle at a time moves through these fields, the prior art methods missort the additional particles because they would be sorted with the sensed particle. The prior art is thus limited to sorting rates of a few hundred cells per second (see the cited Review article, on page 1302 and Scientific American on page 111).
The prior art teaches production of approximately 45,000 droplets per second, but can only sort a few hundred to a few thousand particles per second. Thus, a technique of using multiple "dummy drops" to insure isolation of each cell into a single sorted droplet has been developed. This greatly limits the number of cells that can be sorted in a given time by prior art methods.
Nothing in the prior art teaches or suggests the use of a single electrode to charge and deflect particles, nor is there any suggestion that sorting rates can be increased by the use of a single charge/sort process. Since the sorting rate is critically important to the practical utilization of these devices in biological applications, this is a major problem that has long defied solution. The best prior art devices, for example, would have to operate continuously for about one year to sort an ounce of cells. It may be readily appreciated that this limits their usefulness, especially where the sorting is being done in an attempt to isolate cells that possess statistically rare characteristics.
The claims of issued U.S. Pat. No. 4,097,373 are directed to a single electrode sorter that uses a field emission cathode. In the course of building and testing the device taught by U.S. Pat. No. 4,097,373, the applicant found that, very unexpectedly, a positive charge on the sorting electrode caused about the same amount of deflection as did a negative charge. This effect was unexpected because the field emission cathode establishes a charge on the motivated drop by adding electrons to it. No charge can be added by the positive electrode, thus the inventor is uncertain how this improvement to his basic patent works. It does however, work, by whatever mechanism.